1. Field of the Invention
The present invention relates to an anodic oxide film of a valve action metal typically represented by tantalum or niobium (to be referred to as a chemical conversion film) and an electrolytic capacitor in which it is used.
2. Description of the Related Art
Since chemical conversion films (anodic oxide films) of valve action metals such as tantalum and niobium have satisfactory insulating properties and a high dielectric constant, they are used as the main composite material of capacitors. In particular, since chemical conversion films of tantalum also offer the advantage of stable dielectric properties relative to temperature, they are frequently used for solid electrolytic capacitors.
Although the guaranteed stable operating period of solid electrolytic capacitors differs according to the application, to the extent to which they are used in severe environments, they are required to demonstrate stable operation over a long period of time. One cause of decreased operation including changes in electrostatic capacitance is a qualitative change in the chemical conversion film due to migration of oxygen atoms within the chemical conversion film in the operating environment of the capacitor. The mechanism of this phenomenon is considered to be the result of oxygen atoms in the amorphous structure of the chemical conversion film migrating to the side of the crystalline base metal across the boundary between the chemical conversion film and metal, thereby promoting the formation of crystalline oxide at the boundary (refer to, for example, “Corrosion Science”, Vol. 28, No. 1, pp. 43–56 (1988)).
Conventionally, technology for improving the operating stability of solid capacitors consisted nearly entirely of modifying the structure of the solid capacitor and the materials of the cathode and anode, or technologies relating to their forming methods, while a method for suppressing the migration of oxygen atoms as described above by changing the structure of the chemical conversion film has currently yet to be found.
An object of the present invention is to provide a highly stable chemical conversion film having a structure for suppressing the migration of oxygen atoms within the chemical conversion film. In addition, an object of the present invention is to provide an electrolytic capacitor using this highly stable chemical conversion film that is able to withstand severe usage environments and guarantee stable properties for a long period of time.